DEPARTMENT OF COMMERCE 



Technologic Papers 



or THB 



Bureau of Standards 

S. W. STRATTON. DiRBCTOR 



No. 206 

EFFECT OF HEAT TREATMENT ON 

THE MECHANICAL PROPERTIES OF 

1 PER CENT CARBON STEEL 



BY 



H. J. FRENCH, Physicist 
W. GEORGE JOHNSON, Assistant Physicist 
Bureau of Standards 



DECEMBER 27, 1921 




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DEPARTMENT OF COMMERCE 



Technologic Papers 



OP THE 



Bureau of Standards 

S. W. STRATTON. Director 



No. 206 

EFFECT OF HEAT TREATMENT ON 

THE MECHANICAL PROPERTIES OF 

1 PER CENT CARBON STEEL 



BY 

H. J. FRENCH, Physicist 

W. GEORGE JOHNSON, Assistant Physicist 
I 

Bureau of Standards 



DECEMBER 27, 1921 




PRICE. 15 CENTS 

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-^ n -- O / I 7n 



EFFECT OF HEAT TREATMENT ON THE MECHANI- 
CAL PROPERTIES OF 1 PER CENT CARBON STEEL 

By H. J. French and W. George Johnson 



ABSTRACT 

The effects of varying time-temperatiire relations in heat treatment on tensile and 
impact properties, hardness, and structure of i per cent carbon steel have been 
studied, including (a) effect of temperattu'e variations in hardening, (b) time at 
hardening temperattu-es both above Ag^ and between the Ac, and Acm, transforma- 
tions, (c) effects of tempering steel hardened in different ways and effects of " soaking ' ' 
just under the lower critical range, (d) comparison of oil and water hardening for 
production of definite strengths. 



CONTENTS 

Page 

I. Introduction 93 

II. Previous investigations 94 

1. Annealing 94 

2. Hardening 96 

(a) Quenching temperature 96 

(6) Time at hardening temperattu-es 97 

3. Tempering 98 

(a) Tempering temperattu'e 98 

(b) Time at tempering temperattu'e 99 

III. Materials and methods of testing 100 

IV. Experimental results loi 

1 . Hardening loi 

(a) Effect of quenching in different media loi 

(b) Effect of various oil-quenching temperatures 105 

(c) Time of heating prior to oil quenching 107 

Time at temperature above A^m 108 

Time at temperature between Acj and Acm 109 

2. Tempering in 

(a) Effect of tempering steel hardened in different ways in 

(6) Effect of time in tempering at a temperature slightly below Acx 117 

V. Discussion , 119 

VI. Summary and conclusions 120 

I. INTRODUCTION 

During 1920 there was brought to the attention of one of the 
authors the lack of adequate information in the literature regard- 
ing the most suitable heat treatments for production of the best 
combinations of strength and ductility for i per cent carbon 

93 



94 Technologic Papers of the Bureau of Standards [Voi. i6 

steel. While considerable data are available relating to tensile 
properties of high-carbon alloys, it was not possible to make 
satisfactory comparisons between the results reported by various 
investigators as to the effectiveness of different treatments for 
the purpose in view, due to differences in chemical composition, 
size of specimens treated, and other factors. 

Because of the varied applications of slightly supersaturated 
carbon steels, including tools, dies, bearings, and springs, tests 
were made to correlate properties and structure and determine 
the magnitude of the effects observed with varying time-tempera- 
ture relations in certain heat-treatment operations. At the same 
time treatments resulting in the best tensile and impact properties 
were sought. 

II. PREVIOUS INVESTIGATIONS 

Among the tests previously reported the following are con- 
sidered of particular interest in the treatment of i per cent carbon 
steel or deal with variables studied by the a'uthors. 

1. ANNEALING 

Sargent^ found greatly increased strength with increase in 
annealing temperatures above the critical range and a maxi- 
mum when cooled from 1025° C (1877° F), which was decreased 
about 38 per cent when the temperature was raised to 1150° C 
(2102° F). Maximum ductility was obtained in slow cooling 
from the upper end of the critical range, while temperatures even 
moderately above this reduced the elongation and reduction of 
area to very low values. The accompanying microstructural 
changes were marked, and it was found that after anneaHng at 
915° C (1679° F) the cementite surrounding grains of pearlite 
had all left the boundaries and "gone toward binding groups of 
pearlite crystals into larger compound crystals." 

CampbelP also studied the annealing of i per cent carbon steel 
and found that maximum strength was produced after slow 
cooling from 905° C (1661° F), while the highest ductility and 
lowest strength was obtained when using 760° C (1400° F), 
which is slightly above or at the end of the Aci transformation. 
In general, the inflections in curves for strength and elastic limit 

• George W. Sargent, "A study of the effect of heat treatment on crucible steel containing i per cent 
carbon," Trans. A. T. M. E., p. 303; 1901. 

' William Campbell, "On the heat treatment of some high carbon steels," Proc. Amer. Soc. Test. Mats., 
p. 211; 1906. 



French 1 
Johnsoni 



Heat Treatment of i Per Cent Carbon Steel 



95 



were accompanied by the reverse inflections in curves for elonga- 
tion and reduction of area, showing that an increase in one set 
was obtained at the expense of the other. 

Both Sargent's and Campbell's results are shown graphically 
in Fig. I, while in Table i are given results of tests included in the 
Sixth Report to the Alloys Research Committee' for steel con- 



\A0 



I 

ST 

i- 100 

K 

I 




40 



I 



t 




Ft'K Lies- Camp belt 
m!t1etiLJits - iiryent 



loo 



QOQ SCO lOOO 1100 

flnnealmq lemperaiure ^JOe^.C 

Fig. I. — Effect of different annealing temperatures on the tensile properties of i per cent 

carbon steel 

After Campbell and Sargent, as follows: W. Campbell, On the heat treatment of some high-carbon steels, 
Proc. A. S. T. M., p. 211; 1906. G. W. Sargent, A study of the effect of heat treatment on cnidble steel 
containing one per cent carbon. Trans. A. I. M. E., p. 303; 1901. 

taining 0.95 per cent carbon. Maximum strength was obtained 
in slow cooling from 900° C (1652° F), and the ductility gradually 
decreased with rise in temperatiure from 720° C (1328° F) to 
1 100° C (2012° F). Soaking decreased the strength and in general 
resulted in increase in elongation but decrease in reduction of 
area. 



• Sir W. Roberts- Austen and W. Gowland, "Sixth Report to the Alloys Research Committee: On the 
heat treatment of steel," Froc. Inst. Mecb. Ens.. 1, p. 7; 1904. 



96 



Technologic Papers of the Bureau of Standards 



[Vol. 16 



TABLE 1. — Effect of Time and Temperature in Annealing on the Mechanical Prop- 
erties of 0.95 Per Cent Carbon Steel « 



Annealing temperature 



Time a 1 
temper- 
ature 



Breaking 
strength 



Elastic 
limit 



Elonga- 
tion in 2 
inches 



Reduc- 
tion area 



620° C(U48° F).. 
720° C (1328° F).. 
800° C(1472° F).. 
900° C(1652°F).. 
1100" C (2012° F) . 
1200° C(2192"F). 



Hours 

12 

12 

V2 
12 

V2 

12 

V2 

12 

'A 



Lbs./in.i! 
118 400 
101 800 

88 000 
75 500 

89 500 



Lbs./in.2 
76 600 
51 800 



31 600 
50 400 



114 500 
107 000 
111 300 



80 800 



12 



53 900 



45 000 



Per cent 
15.0 
18.5 
22.0 
23.5 
18.0 



Per cent 
27.0 
27.1 
45.0 
35.4 
28.4 



10.0 

11.9 

7.5 



15.4 

11.0 

9.1 



20.0 



37.5 



o From Sixth Report to the Alloys Research Committee, Proc. Inst. Mech. Eng., 1904, 1, p. 7. Pounds 
per square inch= tons per square inch given in original table, X 2240. 

From the preceding results it appears that slow cooling from 
temperatures just above Acj gives maximum ductility and low 
strength. With rise in annealing temperature the ductility de- 
creases rapidly, and elongation and reduction of area in general 
remain at low values, while the strength reaches a maximum in 
the range about 900° C (1652° F). 

2. HARDENING 

(a) Quenching Temperature. — Brinell* showed that the 
"Hardening Capacity" of carbon steels increased up to 0.45 per 
cent carbon when it became nearly constant, with further increase 
to 0.90 per cent, and thereafter decreased. The steel under con- 
sideration is therefore just above the range of maximum hardening 
capacity, and, as is well known, is quite sensitive to thermal treat- 
ment, having slightly more carbon than required for saturation. 
After quenching in cold water from temperatures above the ther- 
mal transformations it is hard and brittle, while when more slowly 
cooled, as in oil, high strength and somewhat greater ductility 
results. In either case the properties are changed to a marked 
degree by the quenching temperature, whether subsequent tem- 
pering is at low or high temperatures or entirely omitted, as indi- 
cated by results reported by Roberts- Austen and Gowland ^ for an 

* Axel Wahlberg, "Brinell's method of determining hardness and other properties of iron and steel," 
Jr. I. and S. Inst., 1, p. 343; 1901. 
'See note 3. 



French T 
Johnsons 



Heat Treatment of i Per Cent Carbon Steel 



97 



alloy containing 0.95 per cent carbon, which are reproduced in 
Table 2. Based on their tests, the authors conclude that the oil- 
quenching temperature producing the best combination of strength, 
elastic limit, and elongation is in the neighborhood of 900° C 
(1652° F). 

TABLE 2. — ^Effect of Various Methods of Quenching and Tempering on the Tensile 
Properties of 0.95 Per Cent Carbon Steel « 

QUENCHED IN WATER AT 20° C (68° F) 



Quenched from— 



Tempered at- 



Breaking 
strength 



Yield 
point 



Elonga- 
tion in 2 
inches 



Reduc- 
tion ol 
area 



720° C(1328° F).. 
900° C(1652° F).. 
1200° C (2192° F) . 



Lbs./in.' 

125 500 

48 400 

9 400 



Lbs./in.2 



Per cent 
12.0 
Nil. 
Nil. 



Per cent 
24.8 
Nil. 
Nil. 



QUENCHED IN OIL AT 80° C (176° F) 



720° C(1328° F).. 
720° C(1328° F).. 
870° 0(1598° F).. 
1000° C (1832° F). 
800° C(1472°F).. 
900° C(1652°F).. 



350° C (662° F).. 
350° C(662° F).. 
350° C (662° F) . . 
600° C(U12° F). 
600° C(1112° F). 



117 100 


89 500 


13.0 


122 500 


81 700 


12.0 


229 500 


149 400 


7.0 


211 000 


152 700 


5.5 


103 000 


80 000 


12.0 


110 000 


90 000 


17.0 



19.8 
18.6 
17.0 
14.8 
12.9 
28.0 



" From Sixth Report to the Alloys Research Committee, Proc. Inst. Mech. Eng., 1904, 1, p. 7. Pounds 
per square inch=tons per square inch given in original table, X2240. 

Hanemann" reported direct decrease in strength as the water, 
hardening temperature increased from 750 to 1000° C (1382 to 
1832° F) for I per cent carbon steel not subsequently tempered 
whereas for an alloy containing i .33 per cent carbon the strength 
increased until the quenching temperature exceeded 900° C 
(1652° F). 

(6) Time at Hardening Temperatures. — ^AU that is generally 
deemed necessary in the treatment of steel as to time at harden- 
ing temperature is to maintain it for the minimum period required 
for imiform heating throughout. The time actually required to 
meet this condition is known to increase markedly with size, and 
it is also known that with increased mass the temperature at which 
the steel will first harden rises. Porte vin' has shown that the 
effect of time of heating prior to quenching is marked, and that a 
change from 2 to 60 minutes resulted in increased strength and 
hardness as shown below. 

» H. Hanemann, "tjber die Warmebehandlnng der Stable," Stahl and Eisen, 31, p. 1365; 1911 . 
' A. Portevin, "Influence due temps de chaufiage avant la trempe sur les r^sultats de cette operation," 
Rev. Met., 13, p. 9; 1916. 



98 



Technologic Papers of the Bureau of Standards \Voi.i6 



TABLE 3. — ^Effect of Time of Heating Prior to Hardening on the Mechanical Prop- 
erties of 1.08 Per Cent Carbon Steel a 



TimeofheatineatSOO'C 


Maxi- 
mum 
strength 


Elastic 
limit 


Elonga- 
tion in 
100 mm 

(3.94 
inches) 


Reduc- 
tion of 
area 


Brinell 
hardness 
number 




Lbs./in.2 

91 500 

121 000 

125 000 


Lbs-Ziu-S 
78 300 

121 000 

122 000 


Per cent 
10.5 
10.5 
10.5 


Per cent 
17.2 
17.2 
17.2 


600 


?n tn)nutes 


571-652 


60 minutes 


875 







o A. Portevin, "Influence du temps de chauffage avant le trempe sur les resultats de cette operation," 
Rev. Met. (1916), 13, p. 9. Pounds per square inch= kilograms per square millimeter given in original 
tables, X1.433X103. 

3. TEMPERING 

(a) Tempering Temperature. — The effects of tempering on 
the tensile and impact properties of hardened i per cent carbon 
steel have been studied by a number of investigators, including 
Rudeloff,^ Hanemann,^ and Roberts- Austen, and Gowland." 
More recently J. H. Nead" determined the tensile and impact 
properties of such steel quenched in oil from the recommended 
annealing temperature range of the American Society for Testing 
Materials " when followed by tempering at various temperatures, 
but the possibilities for production of high combinations of 
strength and ductility by varying hardening and tempering treat- 
ments have not been fully covered. It appears from Nead's 
results, reproduced in Table 4, that tempering has a relatively 
small effect in reducing the brittleness of the oil quenched steel, 
and that tempering temperatures in the neighborhood of 500° C 
(932° F) are required for a material increase in ductility or decrease 
in tensile strength. The cause of this is undoubtedly in incom- 
plete hardening, as specimens one-half inch or more in diameter 
will not be martensitic throughout after quenching in oil. 

8 M. Rudeloff , ' ■ Untersuchungen Uber den Einfluss des AusglUhens auf die physicalischen Eigenschaften 
von Eisen und Stahdrahten." Stahlund Eisen, 12, p. 63; 1892. 

" See note 6. 

'"See note 3. 

" J. H. Nead, " The egect of carbon on the physical properties of heat-treated carbon steel," Trans. 
Amer. Inst. Min. Eng. 53, p. 218; 1913. Charpy results as related to carbon content of steel. Tests of Metals, 
etc., p. 109; 1916. 

"Year Book,. Amer. Soc. Test. Materials., p. 201; 1914. 



French "] 
Johnsoni 



Heat Treatment of i Per Cent Carbon Steel 



99 



TABLE 4.— Effect of Tempering on the Mechanical Properties of Hardened 1 Per 

Cent Carbon Steel" 
[Samples quenched in oil from 790° C (1454° F)] 



Tempered at— 



375° C (707° F) . 
460° C (860° F).. 
560° C (1040° F) 
650° C (1202° F) 
As Tolled 



Tensile 
strength 



Lbs./in.a 
192 500 
195 000 
201 500 
168 500 
134 000 
152 000 



Yield 
point 



Lbs./in.a 
127 000 
127 500 
111 000 
104 500 
86 000 
86 000 



Elonga- 
tion in 2 
inches 



Per cent 
10.5 
12.5 
12.5 
14.5 
20.0 
9.5 



Reduc- 
tion of 
area 



Per cent 
34.0 
34.0 
40.3 
30.7 
40.3 
13.3 



Brlnell 
hardness 
number 



387 
375 
402 
321 
277 
302 



Impact 

energy 

absorbed 

(Charpy) 



Ft.-lbs./ln.« 



42 
41 
54 
57 
27 



oj. H. Nead, "The effect of carbon on the physical properties of heat-treated carbon steel," Trans. A. I. 
M. E., 53 p. 2i8; 1915. Charpy results as related to carbon content of steel. Tests of Metals, etc., 1916; p. 109. 

(&) Time at Tempering Temperature. — It is well recognized 
that a long time in tempering hardened steel at a given tempera- 
ture is, within limits, equivalent to tempering for a short time at 
a higher temperature. Long-time tempering at a given tempera- 
ture increases the ductility and lowers the strength and hardness, 
but the magnitude of these effects, especially at the lower temper- 
ing temperatures, depends upon the thoroughness of the hard- 
ening. The fact that Hayward and Rajnnond" found relatively 
small effects on the tensile properties of 0.45 per cent carbon 
steel with increased time of tempering was probably due in part 
to incomplete hardening, as pointed out in discussion of their 
results. 

Matthews and Stagg" give detailed data relating to this subject, 
and the!ir values are reproduced below. In part, they state 

TABLE 5.— Effect of Tune in Tempering Hardened Steel « 
[Results are average of 4 check tests after each treatment, carried out on one-hali-inch round tensile test bars. 



Elastic 
limit 



Mazi- 

mxmi 

strength 



Elonga- 
tion 



Reduc- 
tion of 
area 



Brinell 
hardness 



Treatment 



Lbs./hi.> 
228 750 
201 125 
175 000 



Lbs./in.5 
260 137 
214 562 
183 187 



Per cent 

2.5 

11.6 

12.0 



Per cent 



45.4 
49.35 



425 
390 
340 



843° C (1550° F) oil, 437° C (800° F), 8 minutes 
843° C (1550° F) Oil, 437° C (800° F), 20 minutes 
843° C (1550° F) oil, 437° C (800° F), 40 minutes 



« J. A. Matthews and H. J. Stagg, "Factors in hardening tool steel," Trans. A. S. M. E., p. 84s; 1914- 

"* * * time at the drawing temperature has a marked effect. 
The act of breaking down the martensite is progressive and not 
sharply defined. Both time and temperature have their effects." 

"C. R. Hayward and S. S. Raymond, "Effect of time on reheating hardened carbon steel below the 
critical range," Trans. A. I. M. E., p. 517; 1916. 

"J. A. Matthews and H. J. Stagg, " Factors in hardening tool steel," Trans. Amer. Soc. Mech. Eng., 
p. 84s; 1914- 

62081°— 22 2 



loo Technologic Papers of the Bureau of Standards ivoi.id 

Similarly, other investigators have shown the importance of 
the time effect in tempering, notably Barus and Strouhal,^^ who 
used thermoelectric methods; Goerens,^® who studied variations 
in tensile, magnetic, and other properties of low carbon steel; 
Portevin,^^ who carried out hardness tests and microscopic exam- 
ination; and likewise Rudeloff" and Hanemann.^* 

III. MATERIALS AND METHODS OF TESTING 

The steel tested was part of a lot of i by _K inch hot-rolled bars 
of varying carbon contents suppHed by the Carnegie Steel Co., 
Pittsburgh, Pa., and of the following chemical composition: 

Percent 

Carbon i. 04 

Manganese 17 

Phosphorus 017 

Sulphtir 019 

Silicon 14 

After cutting to the desired lengths for tensile and impact 
specimens the steel was normaUzed by heating to 815° C (1500° F) 
for 30 minutes and cooling in still air, thereafter showing the 
following tensile properties and hardness : 

Tensile strength, pounds per square inch 129 900 

Proportional limit, pounds per square inch 55 000 

Per cent elongation in 2 inches 14. o 

Per cent reduction of area 22. 3 

Brinell hardness 217 

Shore hardness 32 

Test samples were machined to slightly larger than the required 
size (approximately yi-mch roimd in reduced section), subjected 
to various heat treatments, and finally ground wet to the form 
and dimensions shown in Fig. 2. Tensile tests were made with a 
50 000-pound testing machine, and a strain gage was used in 
determination of the limit of proportionaHty, while hardness was 
obtained by both the Shore and Brinell methods, by means of a record- 
ing scleroscope and an American Brinell hardness testing machine 
(the latter under standard conditions of 3000 kg load and 10 mm 
ball). Impact specimens of both Izod and Charp}?^ types were 

" Barus and Strouhal, "On the physical characteristics of iron carburets," Bull. 14, U. S. Geol. Survey; 
1885. 

"P. Goerens, "Influence du traitement thermique sur les proprietes de I'acier ecruit," Rev. Met. 
Memoires, 10, p. 1337; 1913. 

" A. Portevin, "Influence du temps de chaufiage avant le trempe sur les resultats de cette operation," 
Rev. Met., 13, p. 9; 1916. 

■8 See note 8. 

" See note 6. 



French T 
Johnsim\ 



Heat Treatment of i Per Cent Carbon Steel 



lOI 



tested in American made Izod and Charpy machines. Prior to 
subjecting the steel to heat-treatment the thermal transformations 
were determined in a manner already described by Scott and 
Freeman. ^'^ The heating and cooling curves so obtained are 
shown in Fig. 3. 

Tensita PuPsptGimeii 







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Fig. 2. — Form and dimensions of test specimens used 

IV. EXPERIMENTAL RESULTS 
1. HARDENING 

(a) Effbct of Quenching in Different Media. — In order to 
throw further Hght on the efifect of several methods of hardening 
I per cent carbon steel when followed by high tempering, such as 
would possibly bring it into suitable condition for structural pur- 
poses, samples were quenched in water, oil, molten lead, and a hot 



*> H. Scott and J. R. Freemen, jr.. Use of a Modified Rosenhain Furnace for Thermal Analysis, Buieatt 
of Standards Scientific Paper No. 348, Oct. 24, 1919. 



I02 



Technologic Papers of the Bureau of Standards \voi. i6 



salt mixture consisting of 2 parts, by weight, of potassium nitrate 
and 3 parts sodium nitrate. The details of these treatments and 
test results are given in Table 6, and it is evident that the surface 
of the treated metal is an important and determining factor in the 
tensile properties obtained. Removal of surface irregularities re- 
sulting from scaling in treatment, including a large part or all of 





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Fig. 3. — Inverse rate heating and cooling curves of l per cent carbon steel 



Ac^rc) 


Ar ^ (° C) 


Beg. 

716 


Max. 

727 


End 

740 


Beg. 

703 


Max. 
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End 

680 



the decarburized surface, results in better combinations of strength 
and ductility, but the steel is very brittle, as shown by the low 
impact values, regardless of the quenching method employed. In 
general, the most marked effect of grinding after treatment is 
found in values of reduction of area, which greatly increase. 



French 1 
Johnsoni 



Heat Treatment of i Per Cent Carbon Steel 



103 



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I04 



Technologic Papers of the Bureau of Standards ivoi. i6 



It appears that the highest strength and ductiHty are obtained 
by cooHng in oil, but a much higher elastic ratio results from 
water quenching. It does not necessarily follow, however, that 
the results reported for any quenching method are the best which 



zoo 



130 180 




c 




^ 






«» 


400 


^ 




X 




«n 


350 


70 


300 


M 



250 50 



200 40 



"^ 



TT5 



600 ees 

Degrees C. 
Oil Quenching Temperature 



eso 



en 



Fig. 4. — Effect of varying oil quenching temperatures on mechanical properties of i per 
cent carbon steel subsequently tempered at Sj8° C 

may be produced, for it is possible that better combinations of 
strength and ductility may be obtained when using higher quench- 
ing temperatures than 788° C (1450° F), as is true when cooling 
in molten lead (Table 6). 



French "1 
JoknsonJ 



Heat Treatment of i Per Cent Carbon Steel 



105 



The structures of this steel after apphcation of these different 
treatments are shown in Fig. lo, and after quenching in water or 
oil, followed by tempering at 538° C (1000° F), the steel is sorbitic. 
When quenched in molten lead or salt, the steel consists of sor- 
bitic pearlite, but the transformation appears to have progressed 
somewhat further to pearlite in the case of the lead quenching. 

(6) Ei^FECT OP Various Oii.-Quenching Temperatures. — 
The effect of different oil-quenching temperatures on the proper- 
ties and structures of steel subsequently tempered at 538° C 
(1000° F) is marked, as shown in Table 7 and Figs. 4 and 11. With 
rise in temperature to 843° C (1550° F) strength and limit of 
proportionality increase and elongation and reduction of area, 
decrease. With further rise in quenching temperature the 
strength factors decrease with practically no change in ductility 
but in all cases the resistance to impact is low. 

TABLE 7. — ^Effect of Varying OU-Quenching Temperahires on the Mechanical Prop- 
erties of 1 Per Cent Carbon Steel, Subsequently Tempered at 538° C 



Specimen 


Heated to 

temperatures 

noted, held 

30 minutes, 

and quenched 

in oil a 


TensUe 
strength 


Propor- 
tional 
Umit 


Elonga- 
tion in 
2 inches 


Reduc- 
tion ot 
area 


Hardness 
number 


Impact energy 
absorbed 


No. 


Brl- 
neU 


Shore 


Charpy 


Izod 


IB 

60B.. 


1 760° C 
1 (1400° F) 


Lbs./in.» 
f 158 000 


Lbs./in.2 
101 000 


Per cent 
13.5 


Per cent 
44.3 


288 


43 


Ft.-lbs. 
2.2; 2.5; 3.0 


Ft.-lbs. 
2.3; 2.2; 2.5 




















Av 
















2.6 


2.3 




















3B 


1 788° C 
J (1450° F) 


J 188 250 i 110 500 






340 
318 


48 
48 


3.0; 3.5; 2.3 
3.0; 3.5; 2.3 


2.0; 2.0; 3.0 


4B 


1 169 000 


113 000 


13.0 


39.3 


2.0; 2.0; 3.0 


Av.. . 




178 600 


111 750 


13.0 


39.3 


329 


48 


2.9 


2.3 








SB 

6B 


1 816° C 
1 (1500° F) 


I 193 600 
1 181 600 


122 000 
116 000 


10.0 
12.5 


29.2 
31.0 


364 
364 


54 
50 


3.1; 3.3; 3.6 










Av.. 




187 600 


119 000 


11.2 


30.1 


364 


57. 


3.3 


* 






1 




7B 

8B.. 


843° C 
(1550° F) 


1 199 000 
1 185 000 


121 000 
119 000 


10.5 
11.5 


29.5 
29.5 


364 
387 


53 
54 


3.1; 3.3; 3.1 


3.1; 3.2; 3.1 








Av 




192 000 


120 000 


11.0 


29.5 


376 


54 


3.2 


3.1 








9B 

lOB.. 


871° C 
(1600° F) 


1 177 900 
1 177 100 


118 000 . 

119 000 


11.5 
12.5 


30.3 
29.8 


345 
356 


46 
50 


3.2; 3.9; 3.9 


3.1; 2.8; 3.1 








Av 




177 500 


118 500 


12.0 


30.0 


350 


48 


3.7 


3.0 







o All samples subsequently tempered at 538° C (1000° F) for 20 minutes and cooled in oil. 



io6 



Technologic Papers of the Bureau of Standards [Voi.i6 



Benedicks ^^ has shown that an increase in temperatiure results 
in more rapid cooling for steel of constant mass, and that the 
metal passes more rapidly through the transformations. His 
results for an alloy containing i per cent carbon are reproduced 
in part below. 

TABLE 8. — Influence of Temperature on Time of Cooling in Quenching 1 Per Cent 

Carbon Steel « 



Weight of specimen in grams 


Quench- 
ing tem- 
perature 


Cooling 
time to 
100° c 


12.5 


°c 

950 
845 
750 
703 
695 


Seconds 
3 07 


12.3 


4.43 


12.4 


4.11 


12.3 


5.73 


12.2 


6.20 


^ 





o C. Benedicks, " Experimental researches on the cooling power of liquids, on quenching velocities, and 
on the constituents of troostite and austenite," Jr. I. and S. Inst. 2, p. 133; 1908. 

It is quite evident, therefore, that a hardening of the matrix 
of the steel under investigation should result with rise in quench- 
ing temperatures and more carbide should be held in solution. 
In this case, however, there is an added factor contributing to 
the increased hardness and strength shown in Fig. 4 in the reten- 
tion of the excess cementite when quenching from above its 
solution temperature range. The rate of cooling in oil when the 
steel is heated to 843° C (1550° F) is not sufficiently rapid to retain 
all this carbide in solution, but an increase to 871° C (1600° F) 
increases the cooling rate through the transformations enough 
to cause retention of the few remaining globules found after the 
first-mentioned treatment. These structures, as well as those 
obtained on using lower quenching temperatures, are shown in 
Fig. II, and because of the tempering at 538° C (1000° F) the 
groundmass is sorbitic. There is free cementite present in good 
proportion until quenching temperatures above 816° C (1500° F) 
are used, while the condition of maximum strength obtained in 
the sample quenched from slightly above the Ken transformation 
is coincident with the minimum proportion of free carbide. Un- 
doubtedly the cementite is first wholly retained in quenching 
from a temperature somewhat below 871° C (1600° F), where the 
first disappearance has been noted, and possibly this condition 

" C Benedicks, " Experimental researches on the cooling power of liquids, on quenching velocities ,and 
on the constituents of troostite and austenite," Jr. I. and S. Inst., 2, p. 153; 1908. 



French T 
Joknsonj 



Heat Treatment of i Per Cent Carbon Steel 



107 



would be coincident with higher strength than shown in Fig. 4. 
However, it is acknowledged that each increase in temperature 
is accompanied by increased grain size. This effect opposes 
whatever beneficial results might be obtained by total retention 
of the cementite and will finally induce greater brittleness and 
lowered tensile strength, and that this has happened in the range 
imder consideration is shown by the sharp decrease in strength 
between samples quenched from 843 and 871° C (1550 and 1600° F). 



200 



130 190 



120 

I 

o 

I 

£ 






f 

c 
u 

h 

■£. 
S 

c 



£ 40 



30 



§20 

I. 

SL 



10 



Prop. Limit 



Brinell, 



Shore 



Tensile ; 



E.[onq. 




2 in. 



4S0 



400 n 

O 

350 GO 



% 50 



\ Z 3 ^ 5 

Ti"me /nhours held ata43'c((5TO°f^ 

FlG. 5. — Effect of time at 843° C on the mechanical properties of i per cent carbon steel 
subsequently oil quenched and tempered at 53^° C 

(c) Time; of Heating Prior to Oil Quenching. — In order to 
further investigate the magnitude of the effects of ' ' soaking ' ' at vari- 
ous hardening temperatures, samples were held for different intervals 
at 760° C (1400° F) and 843° C (1550° F) prior to quenching 
in oil and tempering. The first temperature is just above or 
at the end of Ac 1 , while the latter is slightly above the temperature 
range at which the excess carbide goes into solution. 



io8 Technologic Papers of the Bureau of Standards Woi. i6 

Time at Temperature Above Acm- — ^The time (over 30 minutes) 
for which the steel is held at 843° C (1550° F) when subsequently 
oil quenched and tempered at 538° C (1000° F) has a rela- 
tively small effect on the tensile and impact properties, as shown 
in Fig. 5 and Table 9. A change from 30 minutes' heating to 
I hour does not alter the strength appreciably, but results in 
slightly increased ductility, whereas with increase in time of 
"soaking" from i to 5 hours the hardness, tensile strength, and 
proportional limit gradually increase with accompanying decrease 
in elongation and reduction of area. The steel is very brittle after 
all treatments, and the variations in impact resistance are not 
such as to allow conclusions to be drawn. 

TABLE 9.— Effect of Time at 843° C on the Mechanical Properties of 1 Per Cent 
Carbon Steel Subsequently Oil Quenched and Tempered at 538° C 





Heated to 

843° C 

(1550° F) 

for time 

noted and 

oil quenched; 

tempered 

20 minutes 

at 538° C 

(1000° F) 


Tensile 
strength 


Propor- 
tional 
Umit 


Elonga- 
tion in 
2 inches 


Reduc- 
tion of 
area 


Hardness 
number 


Impact energy absorbed 


Specimen 
No. 


Bri- 
nell 


Shore 


Charpy 


Izod 


7B 

8B 


Hours 

1 . 


Lbs./in.2 
J 199 000 
1 185 000 


Lbs./in.! 
121 000 
119 000 


Per cent 
10.5 
11. S 


Per cent 
29.5 
29.5 


364 
38/ 


53 
53 


Ft.-Ibs. 

1 3.1,3.3,3.1 


Ft.-lbs. 
3.1,3.2,3.1 


Av 


192 000 


120 000 


11.0 


29.5 


376 


53 


3.2 


3.1 




1 ' 




27B 


1 202 100 
1 181 800 


128 000 
113 000 


10.5 
12.5 


27.7 
34.4 


387 
364 


56 
54 






28B 












Av 


191 950 

1 192 350 
I 193 400 


120 500 


11.5 


31.0 


376 


55 








] 2% 






6SB 


121 000 
120 500 


U.O 
9.5 


26.3 
26.6 


376 
387 


55 

57 






29B 












Av 


192 875 


120 750 


10.2 


26.4 


382 


56 








! = 






31B 

32B 


J 207 900 
1 187 000 


129 500 
116 000 


8.5 
10.5 


19.5 
27.0 


477 
364 


56 
59 


3.2,3.2,4.0 


3.1,3.1,3.0 


Av 


197 450 


122 750 


9.5 


23.2 


421 


58 


3.5 


3.1 









The hardening temperature chosen is only slightly above the 
solution temperature of the excess carbide which, according to 
recent determination by Ishiwara,^^ takes place at about 835° C 
(1535° F), and that this reaction proceeds slowly is at once evident 
from examination of the structures shown in Fig. 12. Solution is 

'2 Torajuro Ishiwara, " On the magnetic determinations of Ao, Aj , A2, and A3 points in steels containing 
up to 4.8 per cent carbon," Science Reports of the Tohoku Imperial Univ., 9, No. j, p. 401. 



French "I 
Johnsoni 



Heat Treatment of i Per Cent Carbon Steel 



109 



incomplete at 30 to 45 minutes, whereas m. 2% hours this change 
has progressed nearly to completion, coincident with a marked 
decrease in reduction of area in tensile test. In the sample heated 



5* 

n 

8!. 200 

& 

o 

O 190 



\%Q 



o 



efc 
12(7 (70 



1 10 160 







I 



c 
« 

3) 



40 



30 



4- 
§20 

& 

to 



Tens//? 



Orinell 



Sliorti 




f7ong.2in. 



400 



350 


i. 






300 


60 



250 50 



200 40 



I £ 3 4 5 

Time ih hours held at 7eO''c(l400')=7 

Fig. 6. — Effect of liine at y6o° C on the meckanical properties of l per cent carbon steel 
subsequently oil quenched and tempered at 5J^° C 

for five hours the excess carbide is wholly in solution, which 
partly accoimts for the increased hardness obtained. 

Time at Temperature Between Ac^ and Acm- — ^When this steel is 
"soaked" at 760° C (1400° F), which is slightly above the end of 



no 



Technologic Papers of the Bureau of Standards ivoi. z6 



the ACi transformation, oil quenched and subsequently tempered, 
as in the preceding samples, marked changes in properties result, 
as illustrated in Fig. 6 and Table lo. An increase ia time at hard- 
ening temperature from 30 minutes to 2 hoturs results in increased 
hardness, strength, and limit of proportionality with decrease in 
ductility, particularly as measured by reduction of area. Be- 
tween 2 and 5 hours the changes in properties are negligible, there 
being evident principally a small decrease in strength. 

TABLE 10.— Effect of Time at 760° C on the Mechanical Properties of 1 Per Cent 
Carbon Steel Subsequently Oil Quenched and Tempered at 538° C 



Specimen 
No. 


Time at oU 

quenching 

temperature 

of 760" C 

(1400° F) 


Tensile 
strength 


Propor- 
tional 
limit 


Elonga- 
tion in 2 
inches 


Reduc- 
tion of 
area 


Hardness 
number 


Impact energy absorbed 


BrineU 


Shore 


Charpy 


Izod 


25B 

26B 


Hours 

} 'A 


Lbs./ln.2 
J 186 800 
1 170 000 


Lbs./in.2 

117 500 
108 000 


Percent 
11.0 
11.5 


Percent 
38.1 
41.6 


321 
304 


45 
40 


Ft.-lbs. 
3.1; 2.9; 3.0 


Ft. -lbs. 
2.8; 2.7; 3.0 








Av 




17S 400 


112 750 


11.2 


39.8 


313 


43 


3.0 


2 8 








37B 


1 ■ 


















38B 


1 183 250 


117 000 


10.5 


36.8 


358 


53 












39B 


1 ^ 


j 192 800 
1 198 700 


119 000 

120 000 


9.5 
10.5 


25.2 
29.3 


382 
387 


56 

60 






40B 












Av.... 




195 750 


119 500 


10.0 


27.2 


385 


58 














41B 


5 


f 192 500 
1 192 000 


118 500 
120 000 


8.0 
8.5 


20.9 
24.0 


376 
398 


58 
58 






42B 












Av 




192 250 


119 250 


8.2 


22.4 


387 


58 



































In Fig. 13 are shown the structures of the steel under the 
treatments used, and with increase in time at temperature there 
is more free cementite, and the small globules of this constituent 
appear to combine to form larger ones (microphotographs 135 
and 13/). While these changes are to be noted, a marked differ- 
ence in the etching quaUties of the groundmass is observed, indi- 
cating that the structural changes accompanying the variations 
in tensile properties are partly due to changes in the condition 
of the matrix. Specimens heated for various intervals and oil 
quenched without subsequent tempering were examined under 
the microscope and found to consist of troosto-sorbite, and it 
appeared that the proportion of troostite was greater the longer 
the "soaking." 



/X'L] Heat Treatment of i Per Cent Carbon Steel 1 1 1 

When supplemented by Brinell hardness tests, the results of 
which are given in Table ii, it is evident that the effect of in- 
creasing the duration of heating for hardening results in a notice- 
able increase in hardness as does an increase in temperature. 

TABLE 11.— Effect of Time at 760° C Prior to Oil Quenching on the Hardness of 1 

Per Cent Carbon Steel 



Heated at 760° C (1400° F) for time specified and oil quenched 


BrineU 
hardness 
number 


V^hour 


340 




351 




364 







2. TEMPERING 

(a) Bfi^'Ect op Tempering SteeIv Hardened in Different 
Ways. — ^To supplement available information and correlate the 
tensile and impact properties of one heat of steel tempered at 
different temperatures after hardening in different ways, samples 
were tempered between 316 and 704° C (600 and 1300° F) after 
quenching in oil or in water from just above or at the end of Acj 
and in oil from a temperatiure slightly above the Aom transforma- 
tion. 

The results obtained in tests of steel quenched in water from 
788° C (1450° F), followed by tempering between 538 and 704° 
C (1000 and 1300° F), are given in Fig. 7 and Table 12, and it is 
noted that the most rapid decrease in strength occurs when the 
tempering temperature is raised from about 538 to 649° C (1000 
to 1200° F), while at the same time the elongation is almost 
doubled and reduction of area materially increased. 



112 Technologic Papers of the Bureau of Standards voi. i6 




550 GOO OSO 

Degrees C. 

Tampering Temperaturs 



700 



Fig. 7. — Effect of tempering on the mechanical properties of I per cent carbon steel first 

quenched in water from 788° C 



French T 
Johnsonj 

TABLE 



Heat Treatment of i Per Cent Carbon Steel 



113 



12. — Effect of Tempering on the Mechanical Properties of 1 Per Cent Carbon 
Steel First Quenched in Water from 788° C 



Specimen 
No. 


Held 20 

minutes at 

tempering 

temperature 

indicated and 

air cooled 


Tensile 
strength 


Propor- 
tional 
limit 


Elonga- 
tion in 
2 inches 


Reduc- 
tion 
of area 


Hardness 
number 


Impact energy 
absorbed 


Brinell 


Shore 


Charpy 


Izod 


51B 

52B . 


1 538° C (1000° 
1 F) 


Lbs./in.2 
J 165 500 
1 156 200 


Lbs./ln.2 
143 000 
132 000 


Per cent 
11.0 
12.5 


Per cent 

27.7 
34.6 


340 
340 


56 
53 


Ft.-lbs. 
2.5, 3.1, 3.2. 


Ft.-lbs. 
2.2, 2.2, 2.0 








Av . 




160 850 


137 500 


11.8 


31.2 


340 


54 


2.9 


2.1 










53B 


1 649° C (1200° 
1 F) 


( 


106 509 
106 000 


17.0 
22.0 


47.4 
48.6 


255 
255 


43 

43 






54B 


1 118 800 


3.2,2.6 


2.3, 2.0, 2.1 


Av 






106 250 


19.5 


48.0 


255 


43 


2.9 


2.1 












55B 


1704° C (1300° 
1 ^) 


J 108 400 
1 108 700 


91 000 
95 000 






223 
220 


34 
34 


5.7,10.7, 8.5 . 
3.0, 2.9, 2.8. 


11.0, 5.5, 7.7 


56B 


25.0 


52.9 


2.0, 2.4, 2.3 


Av 




108 650 


93 000 






222 


34 


5.6 


5.2 














67B 

68B 


1 760° C (1400° 
f F) 


J 124 400 
1 115 800 


68 000 
67 500 


16.5 
18.5 


37.0 
44.6 


255 
269 


34 
39 


3.2, 2.3, 2.7. 


2.6, 2.4, 2.2 








Av 




120 100 


67 750 


17.5 


40.8 


262 


36 


2.7 


2.4 











The mechanical properties-tempering temperature curves shown 
in Fig. 8 are based on results given in Table 13 for steel quenched 
in oil from 788 or 843° C (1450 or 1550° F) and are of the same 
general type. Samples quenched at the highest temperature 
show the highest strength with lowest elongation and reduction 
of area throughout than those quenched from 788° C (1450° F), 
or, in other words, a higher tempering temperature is required 
when using the highest quenching temperature to produce a given 
strength. 



114 



Technologic Papers of the Bureau of Standards [voi. z6 




500 

D«gra«3C 
Temporiti^ Temparoture 



Fig. 8. — Effect of tempering on the mechanical properties of I per cent carbon steel, first 

oil quenched from either ^88 or 843° C 

Dotted lines represent samples quenched from 788° C 



French 1 
Joknsoni 



Heat Treatment of i Per Cent Carbon Steel 



"5 



TABLE 13. — Effect of Tempering on the Mechanical Properties of 1 Per Cent Carbon 
Steel, First Oil Quenched from 788° C or 843° C 

STEEL HEATED FOR 30 MINUTES AT 788° C (1450° F) AND OIL QXTENCHED 



Speci- 
men 


Held 30 min- 
utes at temper- 
ing temperature 
indicated and 
oil quenched 


Tensile 
strength 


Propor- 
tional 
limit 


Elonga- 
tion in 2 
inches 


Reduc- 
tion o{ 
area 


Hardness 
number 


Impact energy absorbed 


No. 


Brinell 


Shore 


Charpy 


Izod 


UB 

12B 


316° C (600° F) 


Lbs./in.2 
197 000 
206 000 


Lbs./in.2 
122 000 
130 000 


Per cent 
8.0 
9.5 


Per cent 
24.8 
32.0 


364 
386 


52 
56 


Ft.-lbs. 
2.2,3.1,3.3 
4.8,7.5,4.8 


Ft.-lbs. 
2.7,2.1,3.3 






Av.... 




201 500 


126 000 


8.8 


28.4 


375 


54 


4.3 


2.7 








3B 


538° C (1000° F) 


188 250 
169 000 


110 500 
113 000 






340 
318 


48 
48 


3.0,3.5,2.3 
3.9,3.9,4.8 


2.0 2.0,3.0 


4B 


13.0 


39.3 








Av- 




178 600 


HI 750 






329 


48 


3.5 


2.3 












133 

14B... 


|649°C(1200°F) 


140 700 

141 800 


93 000 
91 500 


18.0 


45.3 


278 
277 


41 
41 


3.0,3.1,3.0 
3.9,4.4,3.9 


2.3,2.0,2.4 










Av.. 




141 250 


92 250 






278 


41 


3.5 


2 2 












59B 

16B 


|704°C(1300°F) 


115 800 
117 800 


72 500 
82 000 


20.0 
21.0 


49.8 
47.8 


235 
241 


36 
36 


2.8,2.1,2.2 
4.8 


3.5,3.7,4.0 
2.6,2.1,2.1 


Av.... 




116 800 


77 250 


20.5 


48.7 


238 


36 


3.0 


3.0 








17B 


|760°C(1400°F) 


171 700 
1 171 600 


108 000 
110 000 


12.0 
11.5 


40.4 
42.3 


302 
286 


42 
40 






18B 












Av.... 




171 650 


109 000 


11.7 


41.3 


294 


41 















STEEL HEATED FOR 30 MINUTES AT 843° C (1550° F) AND OIL QUENCHED 



61B 

62B.. 


J316°C(600°F) 


r 228 000 
1 228 200 


145 000 
147 000 


8.0 
8.0 


20.3 
20.3 


512 
470 


66 
64 


3.1,3.2,3.0 
8.0,2.2 


4.0,2.7,3.8 






Av.... 




228 100 


146 000 


8.0 


20.3 


491 


65 


3.9 


3.5 








7B 

8B 


[538° C (1000° F) 


( 199 000 
1 185 000 


121 000 
119 000 


10.5 
11.5 


29.5 
29.5 


364 
387 


53 
54 


3.1,3.3,3.1 


3.1,3.2,3.1 








Av.... 




192 000 


120 000 


11.0 


29.5 


376 


54 


3.2 


3.1 








22B.. 


|649°C(1200°F) 


f 


90 500 
98 500 


17.5 
15.0 


45.6 
41.0 


277 
290 


42 
42 


3.9,4.8,4.8 


3.5,3.5,3.5 


63B 


1 144 700 








Av.... 




144 700 


94 500 


16.2 


43.3 


284 


42 


4.5 


3.5 








23B 

24B.. 


|704°C(1300°F) 


r 125 200 
t 140 200 


86 000 


18.8 
17.0 


46.7 
40.2 


260 
248 


40 
40 


3.3,3.0 
9.6,6.6,4.8 


3.0,2.8,3.0 
4.5,5.5,6.0 








Av.... 




132 700 


86 000 


17.9 


43.4 


254 


40 


5.5 


4.1 








25B 

26B 


760° C (1400° F) 


J 186 800 
I 170 000 


117 500 
108 000 


11.0 
11.5 


38.1 
41.6 


319 
332 


45 
40 


3.1,2.9,3.0 


2.8,2.7,3.0 








Av.... 




178 400 


112 750 


11.2 


39.8 


326 


43 


3.0 


2.8 









ii6 



Technologic Papers of the Bureau of Standards ivoi. i6 



By interpolation of the graphs giving variations in tensile 
properties of the tempered steel hardened in different ways it 
is possible to compare the various treatments applied in order 
to produce a given strength, and the results obtained from this 
approximation are given in Table 14. 

TABLE 14.— Comparison of Oa and Water Hardening of 1 Per Cent Carbon Steel 
for Production of Definite Strengths 

[Approximate values by interpolation] 



Strength 
chosen in 


Method of 
hardening 


Tempering 
required 


Propor- 
tional 
Umit 


Elastic 
ratio 


Elonga- 
tion in 2 
Inches 


Reduc- 
tion of 
area 


Hardness number 


pounds per 
square inch 


Brinell 


Shore 


200 000 

160 000 

135 000 

120 000 


843° CoU 

788° C oil.. . . 
843° CoU.... 
788° Coil.... 
788° C water. 
843° Coil.... 
788° Coil.... 
788° C water. 
788° Coil.... 
788° C water. 


489° C ( 912° F) 
331° C ( 628° F) 
611° C (1132° F; 
591° C (1095° F) 
540° C (1004° F) 
690° C (1274° F) 
661° C (1222° F) 
608° C (1126° F) 
696° C (1284° F) 
648° C (1198° F) 


Lbs./in.2 

125 000 

125 000 

102 500 

101 500 

136 500 

88 000 

87 500 

118 000 

78 000 

107 000 


Per cent 

62.5 
62.5 
64.0 
63.5 
85.3 
65.1 
64.8 
87.3 
65.0 
89.0 


Per cent 
10.5 
10.0 
14.0 
15.5 
12.0 
19.5 
18.5 
17.0 
20.0 
19.0 


Per cent 
27.5 
29.0 
38.5 
42.2 
32.0 
48.0 
45. 5 
42.0 
48.0 
48.0 


400 
370 
312 
302 
342 
260 
265 
290 
240 
260 


56 
53 
46 
44 
54 
41 
39 
47 
37 
43 



For the production of given strength by heat treatment a 
slightly higher tempering temperature is required after oil 
quenching from just above the ACi transformation than when the 
steel is water quenched from the same temperature. 

Likewise a higher temperature is required when oil quenching 
from above the A^m transformation than when similarly cooled 
from just above Ac^ in order to produce the same strength. In 
general, this difference is less as the temperatures at which the 
steel is tempered increases. 

Water-quenched steel subsequently tempered to show the same 
strength as that quenched in oil and tempered has a very much 
higher proportional limit than the latter and also greater hard- 
ness as meastued by the Brinell and Shore methods. As the 
tempering temperature approaches the lower critical range these 
differences at first decrease and then slightly increase. While 
the difference between the hardness of water and oil quenched 
steels is well knoAvn, it is interesting to note the magnitude of 
this difference for the steel tmder consideration and the fact that 
it is maintained over a wide range of tempering. 



to^JL] Heat Treatment of i Per Cent Carbon Steel 117 

In view of the slight differences in tensile properties between 
the steel quenched in oil from 788° C (1450° F) and that from 
843° C (1550° F) the lower temperature is recommended. If the 
excess carbide exists in plate form, due to a previous high heating, 
it may readily be refined by normalizing. A preliminary quench 
from above Acm, however, appears preferable. Water hardening 
is the best method for the production of strengths in the neighbor- 
hood of 120000 lbs. /in. ^ requiring a tempering of about 649 to 
710° C (1200 to 1300° F), providing always that the size and 
shape of the material is such as to allow the use of a drastic 
quenching medium. 

The structures of samples subjected to the various tempering 
treatments are shown in Figs. 14 and 15 and are of the usual 
type. After short- time tempering at 316° C (600° F) the steel is 
troosto-sorbitic, and as the tempering temperature increases there 
is a gradual transition of the groundmass to its more stable form. 
The water-quenched steel, however, has the finest structure re- 
gardless of the tempering temperature used. 

(6) Effect of Time in Tempering at a Temperature 
Slightly Below ACj. — ^The effect of "soaking" at 704° C 
(1300° F) when followed by quenching in oil on the properties 
of oil-hardened steel is shown in Fig. 9 and Table 15. A change 
from one-half hour to five hours has reduced the hardness con- 
siderably, the strength by about one-third, and markedly in- 
creased the ductility. As shown in Fig. 16, there is very little 
change in the character of the excess carbide with increased 
heating time, so that the change in properties is a result of a 
distinct softening of the matrix. Comparison of the tensile prop- 
erties shown in Fig. 9 with results obtained on normalized steel 
at once shows the effectiveness of temperatures just tmder the 
lower transformation for softening this alloy. 



ii8 



Technologic Papers of the Bureau of Standards [Voi. ic 



. 130 



m 

I. 
a. 



leo 



no 



iioo 

c 
«i 

3> 90 
J) 
'S 
^ 80 

i 70 



« 60 



I- 50 




70 



60 



50 



*: 40 



1. 
^ JO 



£0 



10 






500 



250 t 
200 50 

ISO 40 
.£ 30 



I 2 3 4 5 

Time in hours held ai T04''C0300'F) 

Fig. 9. — £^ec< o/" time at a temperature slightly below Acj on ike mechanical properties 
of I per cent carbon steel previously oil quenched from 843° C 

Samples were oil quenched from the tempering temperature 



Technologic Papers of the Bureau of Standards, Vol. 16 




Fig. io. — Alicrostnicture of I per cent carbon steel quenched frotn j88° C in different 
media and tempered at §j8° C. X500 

(a) Quenched in water 

(b) Quenched in oil 

(c) Quenched in molten lead at 538° C 

(d) Quenched in molten sodium and potassium nitrates at 338° C 
All samples etched with 2 per cent nitric acid in alcohol 



Technologic Papers of the Bureau of Standards, Vol. 16 




Fig. II. — Micro structure of i per cent carbon steel oil quenched fro^n various 
temperatures and subsequently tempered at jj8° C. 'X.,')00 

(o) 760° C (1400° F) (c) 616° C (1500° F) (f) 8-1° C (1600° F) 

(6) 788° C (1450° F) (d) 843° C (1550° F) 

Samples etched with 2 per cent nitric acid in alcohol 



Technologic Papers of the Bureau of Standards. Vol. 16 




Fig. 12. — Microsiructure of I per cent carbon steel held at 843° C for various times 
and subsequently oil quenched and tempered at 538° C. X500 

(a) 4s minutes (6) 2K hours (c) 5 hours 

Samples etched with 2 per cent nitric acid in alcohol 



Technologic Papers of the Bureau of Standards, Vol. 16 











^^i*^*c^ ^'• 












i <» "» . 


«. •-..'fc .•. 




^ » 


■>. ."•" 




• < 


. •• &' 


:-f 




. , •-:^- «». 


.. •■* 


* 








' • 5..V , 






Fig. 13. — Microstructure of I per cent carbon steel held at '/6o° C for various times 
and subsequently oil quenched and tempered at 538° C. X^oo 

(a) ^ hour (6) 1 hour (c) 2 hours (d) 5 hours (c) K hour (/) 5 hours 

Samples (e) and (/) etched with boiling sodium picrate. Other samples etched with 2 per cent nitric 
acid in alcohol 



French l 
Joknsonj 



Heat Treatment of i Per Cent Carbon Steel 



119 



TABLE IS.— Effect of Time Slightly Below Aci on the Mechanical Properties of 1 
Per Cent Carbon Steel, Previously Oil Quenched from 843° C 



Speci- 
men No. 


Heated at 

704° C 
(1300° F) for 
time speci- 
fied and 
then oil 
quenched 


Tensile 
strength 


Propor- 
tional 
limit 


Elonga- 
tion in 

2 
Inches 


Reduc- 
tion of 
area 


Hardness 
number 


Impact energy 
absorbed 


Brin- 
eU 


Shore 


Cliarpy 


Izod 


23B 

24B.. 


Hours 

} ^ 


Lbs./in.» 

f 126 200 
I 140 200 


Lbs./in.2 
86 000 


Per cent 
18.8 
17.0 


Per cent 

46.7 
40.2 


260 
248 


40 
40 


Ft.-lbs. 
3.3,3.0 


Ft.-lbs. 
3.0,2.8,3.0 
4. 5 5 5 6 










Av 




133 200 


86 000 


17.9 


43.4 


254 


40 


3.2 


4.1 








34B 


1 


f 121 500 
1 113 200 


79 000 
81 500 






241 
235 


39 
39 






64B 


19.0 


48.4 












Av 




117 350 


80 250 






238 


39 


















35B 

36B 


5 


f 86 100 
1 93 900 


54 500 
72 000 


33.0 


61.3 


156 
207 


32 
34 


2.8,8.4,2.5 


2.6,2.3,3.9 












Av 




90 000 


63 250 






182 


33 


4.6 


2.9 













V. DISCUSSION 

As far as the tensile and impact properties of heat-treated 
I per cent carbon steel are concerned certain facts deserve em- 
phasis in connection with the tests previously described. The 
steel remains extremely brittle, even though good combinations 
of tensile strength and ductility are obtained by suitable treat- 
ment. Such low resistance to impact makes the surface condi- 
tion of the metal of great importance, so that moderately sharp 
comers and even scratches may greatly effect the behavior of 
the steel in service. 

It is extremely sensitive to heat treatment. Small changes in 
the hardening temperature produce large differences in tensile 
properties, though after quenching in a given medium as in oil 
such differences may be largely removed by varying the temper- 
ing temperature. 

To retain all excess cementite in solution when quenching in 
oil requires a relatively high temperature. The lowest tempera- 
ture at which this may be brought about is, among other factors, 
dependent upon the time of heating prior to hardening and is 
lower the longer the "soaking," but must be at or above the 
range of solution of this carbide. Between ACj and A^^ the 
excess cementite spheroidizes and coalesces to form larger glob- 
ules, which are retained in the free state in the oil-quenched steel. 



I20 Technologic Papers of the Bureau of Standards ivoi. lO 

At temperatures slightly below Ac^ long-time heating when fol- 
lowed by cooling in oil produces no change in the condition of 
the cementile which may be readily observed under the micro- 
scope. 

In this connection attention is directed to a recent study of the 
formation of spheroidal cementite by Honda and Saito.^^ The 
authors conclude that spheroidization of granular pearlite takes 
place below Acj if held at temperature for a sufficiently long time, 
but that spheroidization of lamellar pearlite can not proceed until 
ACj has been reached or exceeded. If the maximum temperature 
in heating exceeds a certain limit above this formation, the cemen- 
tite appears as a lamellar pearlite on cooling, and therefore no 
spheroidization will take place. 

The tests carried out in this investigation further indicate the 
importance of time at temperatures between ACj and Ac„ (Fig. 
6) . The sensitivity of i per cent carbon steel in this temperature 
range (within which it is ordinarily heated for hardening) to 
varying time-temperature relations is great, and the data ob- 
tained show why widely different results may so readily be 
obtained in the application of the heat-treated product. 

VI. SUMMARY AND CONCLUSIONS 

Based on the tests made under conditions previously described, 
the following conclusions are drawn : 

1. The most suitable oil or water quenching temperatiue for 
steel which is subsequently to be tempered at relatively high 
temperatures is slightly above the end of the Acj transformation. 

2. With increase in oil-quenching temperature for steel subse- 
quently tempered at 538° C (1000° F) hardness, strength, and 
limit of proportionality increase and maximum values are obtained 
after quenching from 843° C (1550° F) which is coincident with 
retention of all but a small portion of the excess cementite. A 
higher quenching temperature results in decreased strength. 

3. When this steel is subsequently to be tempered to produce 
tensile strength in the neighborhood of 120000 lbs. /in* water 
quenching is to be preferred on account of the higher elastic ratio 
produced, always assuming that the size and shape of material is 
such as to allow drastic treatment. If higher strength is desired 
(which condition requires lower tempering), oil quenching from 

•• K. Honda and S. Saito, " On the formation of spheroidal cementite," Jr. I. and S. Inst., 2, p. 261; 1930. 



Technologic Papers of the Bureau of Standards, Vol. 16 





Fig. 14. — Micro siruciure of I per cent carbon steel oil quenched from y88° C or 843° C 
and subsequently tempered at different temperatures . X500 



(a) Quenched from 788° C 

(b) Quenched from 788° C 

(c) Quenched from 788° C 
(rf) Quenched from 843° C 
(e) Quenched from 843° C 
(/) Quenched from 843° C 

All samples etched with 



and tempered at 316° C (600° F) 
and tempered at 649° C (1200° F) 
and tempered at 704° C (1300° F) 
and tempered at 316° C (600° K) 
and tempered at 649° C (1200° F) 
and tempered at 704° C (1300° F) 
2 per cent nitric acid in alcohol 



Technologic Papers of the Bureau of Standards, Vol. 16 




Fig. 15. — Microstructure of I per cent carbon steel water quenched frotn ^88° C and 
subsequently tempered at different temperatures . Y^soo 

(a) 538° C (1000° F) (b) 649° C (1200° F) (c) 704° C (1300° F) 

Samples etched with 2 per cent nitric acid in alcohol 



Technologic Papers of the Bureau of Standards, Vol. 16 




•?^. -J ■-*• Vji . ■-,-•'.< 



•^. 



.'•*^. 



i'--.-* 






:?. 









5?a' - •-• * 









^iBJf. 



>:*:^^>^ 



.♦»■ A 





















"^J. 



i,.'» 



.^ ■ 






Fig. i6. — Microstructure of I per cent carbon steel oil quenched from 843° C and 
subsequently teyyipered for various times at a temperature slightly below Ac^. Xjoo 

(a) I hour (6) 5 hours (c) i hour (rf) 5 hours 

(0) and (6) etched with 2 per cent nitric acid in alcohol; (c) and (.d) etched with boiling sodium picrate 



jlZ'sL] Heat Treatment of i Per Cent Carbon Steel 121 

just above the Ac, transformation will give slightly better combi- 
nations of strength and ductility but with lower elastic ratio than 
are obtained by water hardening. 

4. A lower tempering temperature is required after water harden- 
ing than when cooling in oil from the same temperature in order 
to produce the same strength. This difference in temperature in 
general increases the higher the strength required (the lower the 
tempering temperatures) . 

5. When samples are quenched in water and others quenched in 
oil and all so tempered as to produce the same strength, the water- 
quenched steel will have the highest hardness as determined by 
the Brinell and Shore methods. 

6. While good combinations of tensile strength and ductility 
may be obtained by tempering at the higher tempering tempera- 
tures, the steel is brittle and has low resistance to impact. This 
is shown by the much higher combinations of strength and ductihty 
obtained in tensile tests when the steel is ground to size after 
heat treatment and by the low Charpy and Izod values obtained 
in all cases. 

7. Increased time at hardening temperatures results in increased 
hardness, strength, and limit of proportionality, and decreased 
ductility, as does rise in temperature. 

8. Long-time heating just under the lower critical range results 
in a material softening of the steel, equivalent to a decrease of 40 
per cent in Brinell hardness and 20 per cent in Shore hardness 
when the time at temperatvire is increased from 30 minutes to 5 
hours. Short-time heating in this temperature range results in a 
softer steel than that air cooled from above the transformations. 

9. The changes in physical properties which have been described 
are in all cases coincident with well-defined structtual changes. 

Acknowledgment is made to T. E. Hamill, laboratory aid of the 
Bureau of Standards, for assistance in carrying out the various 
heat treatments and physical tests involved. 

Washington, June 30, 1921. 



^^SmWW^^^p^i^;^-''-^ 



